Conventionally, information on the accurate position of terminal equipment during the connection between the terminal equipment and a base station has not generally been considered crucially important. However, in planning of radio systems such as cellular telephone systems, the distance of the terminal equipment from the base station must be taken into account, with particular attention being paid on the variation of the distance during connection. This is vital especially in time division digital cellular systems in which transmission is carried out in bursts during a given transmit time slot. In order to prevent bursts transmitted at the same carrier frequency in consecutive time slots from over-lapping in the received signal at the base station, the bursts transmitted by each individual mobile terminal are extended by a guard time of a certain duration at the end of each burst. Such a guard time is necessary, because the mobile terminals transmitting at the same radio carrier, but in consecutive time slots, are located at random distances from the base station, whereby also the propagation time of radio waves from the base station to the mobile terminal will vary from time slot to time slot. The guard time is determined so that the base station, within the timing margin of the transmit/receive frame, measures the arrival time difference between the burst transmitted by the base station and the burst received from the mobile terminal and subsequently computes a timing advance (TA), after which the base station transmits the value thereof to the mobile terminal. Thus, the base station dynamically controls the transmit time of each mobile terminal on the basis of bursts received therefrom. In a GSM system, the timing advance may be assigned values in the range 0-233 ms, which in relation to the time slot overall duration means that a mobile terminal operating with the maximum timing advance cannot be farther away than 35 km from the base station.
As it is generally known, the location of an emitting signal source always requires more than one receiver, spatially separated from each other. Methods of signal direction estimation are based on the estimation of time differentials needed by the same signal to reach the different receivers. Estimation of received signal arrival time differences is performed by correlating the output signals from different receivers. The intersignal delay which gives the maximum value of the correlation function represents the estimate of maximum likelihood for the arrive time difference, provided that said value of the correlation function exceeds a preset threshold value.
In a cellular radio system, the above-described procedure is carried out so that the transmit signal of a mobile terminal is received at a plurality of base stations, or alternatively, at a base station equipped with an array of antenna elements with individual receivers. Then, the received signals obtained from the different base stations or an antenna array of a single base station can be correlated with each other in order to determine the arrival time differences, and therefrom, the bearing angle of the received signal.
These conventional approaches are hampered by the computational complexity involved in relation to the accuracy offered by them.
However, in some cases it is a desirable property of the network to provide maximally exact information on the location of the mobile terminal. One such case is encountered when it is required to restrict the movement of the mobile terminal to occur within the coverage area of a given cell only or when a more detailed scheme of call charging rates is desired.
One approach to the location of a mobile terminal is presented in FI Pat. Appl. No. 963,382, filed in Aug. 30, 1996, not yet public at the filing date of the present application. This method uses an estimated value of the timing advance, which is a readily available value computed by the communications system, proportional to the distance between the terminal equipment and the base station. While the estimate of the timing advance computed for the distance between the serving base station and the terminal equipment gives only a rough estimate of the terminal equipment's distance from the base station, but the estimate of the terminal equipment timing advance computed for a greater number of base stations already gives the terminal equipment location with a relatively good accuracy. The latter technique is supported by the OTD (Observed Time Difference) property of Phase 2 of the GSM standard that directly gives the propagation time differences of the signals from the mobile terminal to the different base stations. Cited patent application further suggests that, in CDMA (Code Division Multiple Access) systems, the mobile terminal location can be determined by correlating the pilot signals transmitted by the base stations.
The location of terminal equipment is especially important also in authority networks. These networks are private cellular networks which are particularly designed to operate under different distress situations, too. Herein, it is important that the network can identify the location of the terminal equipment without the need for the mobile terminal operator to verbally report his location. In open networks, the mobile terminal location information would also be a desirable property in emergency call situations, because the emergency call originator does not necessarily always know his exact location.
It is an objective of the present invention to provide a mobile terminal location method which is characterized by simple computation routines and maximum utilization degree of the properties of existing communication networks.
The goals of the invention are achieved by virtue of the specifications expressed in the appended independent claims.